OSPF intra area priority

[dsvunicredit]

Hello

Question to a OSPF guru

Why does router ID 10.255.255.12 chooses intra-area link even with maximum ip ospf cost in the folowing schema (for better resolution pls. see the attached one):

What is the workaround?

Tunnel between RID 48 and RID 11 is not acceptable.

Thanks!

[samavedula_rama · Accepted solution]

Even before OSPF starts considering COST for best path selection, it will use the following hierarchy.

1) Intra-Area   -- O Routes

2) Inter-Area   -- o IA Routes

3) External type 1 - O E1

4) External Type2 - O E2

If matching criterion is same, then cost is considered.

The best option would be to use another routing protocol and tag those routes with lesser DISTANCE than 90 [OSPF AD]

[samavedula_rama]

oops, I meant 110 OSPF AD

[dsvunicredit]

Rama,

Is the behaviour documented in a worldwide standard (RFC, etc) or it's cisco proprietary?

What will be potential impact to make equal AD for inter and intra OSPF types?

Thanks

[samavedula_rama]

I am pretty sure this is an RFC standard although I don't see it in the RFC doc. Atleast Cisco OSPF design does not say it is proprietory.

[dsvunicredit]

I've added command

distance ospf inter-area 70 intra-area 70 external 70
without any visible result...

The ospf database on the RID 10.255.255.12 shows only the one entry

RID12#s ip os dat | i tate|10.0.0.0
                Router Link States (Area 1014)
                Net Link States (Area 1014)
                 Type-7 AS External Link States (Area 1014)
10.0.0.0        10.255.255.11   1307        0x8000034B 0x000C58 333

May be this is the reason...

But why does the RID 10.255.255.48 not redistribute route to 10.0.0.0 to the 1024 area?

RID48#s ip os dat | i tate|10.0.0.0
                 Type-7 AS External Link States (Area 1003)
10.0.0.0        10.255.255.11   1621        0x8000034B 0x000C58 333
                Summary Net Link States (Area 1014)
                Type-7 AS External Link States (Area 1014)
10.0.0.0        10.255.255.11   1624        0x8000034B 0x000C58 333
                Type-5 AS External Link States
10.0.0.0        10.255.255.11   1624        0x800039F7 0x00456F 333
10.0.0.0        10.255.255.107  574         0x8000034D 0x007E36 333

[samavedula_rama]

That command would work if the router learns routes from multiple protocols or multiple OSPF processes.

If OSPF is your only choice, then you cannot make an Inter area route preferred over an Intra Area route unless you create
another OSPF process.

If not, use another protocol like BGP or EIGRP over tunnel.

I would say use EIGRP over the tunnel and then redistribute static/connected into it. Then OSPF would be preferred since External EIGRP
has an AD of 170, which is greater than OSPF.

[cadet alain]

Hi,

This is an external N1 route but is it an E1 route in area 0? Can you do a sh ip ro for the prefix on the router with RID 48

Maybe it is also an external type 1 but with a higher metric.

Regards.

Alain.

[dsvunicredit]

Hi, Cadetalain

Yes, N1

RID48#show ip ro 10.0.0.0 255.255.255.0
Routing entry for 10.0.0.0/24
  Known via "ospf 200", distance 110, metric 32
  Tag 333, type extern 1
  Last update from 172.16.16.61 on Vlan777, 01:13:38 ago
  Routing Descriptor Blocks:
  * 172.16.16.61, from 10.255.255.11, 01:13:38 ago, via Vlan777
      Route metric is 32, traffic share count is 1
      Route tag 333

Thanks

[cadet alain · Accepted solution]

Hi,

on RID 48 the route is E1 and as Area 104 is NSSA it won't accept OSPF external routes, I think this is the reason why the router is only receiving the N1

route even though its metric is higher.

Regards.

Alain.